1. Field of the Invention
The present invention relates to a displacement detection device for a portable apparatus and, more particularly, to a device that simplifies the user interface of the portable apparatus.
2. Description of the Related Art
As is known, in the last few years, with the increasing use of portable electronic devices (such as laptops, Personal Data Assistants (PDAs), digital audio players, cellphones, digital video cameras, and the like), the need has arisen to simplify the use of such devices and, in particular, to simplify the user interface. In this regard, of particular interest is the possibility of activating given functions or programs of the portable devices with a simple movement imparted thereon by the user. Consider, for example, navigation within a list of options or an address book of a cellphone by simply tilting it or moving it in the direction in which is it is desired to scroll the list or the address book, or again to the possibility of adapting the display of an image on a PDA by simply changing the physical orientation, such as the inclination thereof.
In some portable devices it has been proposed to integrate a displacement detection device, which enables activation of functions or programs upon detection of a movement imparted by the user.
Displacement detection devices of a known type generally include an accelerometer of a linear type, which can be manufactured using the semiconductor technology (so-called MEMS, or micro-electro-mechanical systems). The linear accelerometer is used for measuring the accelerations acting on the portable apparatus so as to determine, with an appropriate conditioning electronics, the direction in which the portable apparatus has been displaced.
As is known, and shown in FIG. 1, a linear accelerometer 1 of a MEMS type includes a sensing element 2, which detects acceleration and generates an electrical signal correlated to the detected acceleration, and a conditioning electronics for conditioning the electrical signal, which generates an output signal. Vout and typically includes a charge integrator 3 and a gain and noise-cancellation stage 4, which uses, for example, the correlated-double-sampling (CDS) technique in order to eliminate the noise.
As shown in detail in FIG. 2, the sensing element 2 includes a stator, of which only first and second fixed electrodes 5a, 5b are shown, and a rotor, made up of a mobile mass 6 and mobile electrodes 7 fixed to the mobile mass 6. Each mobile electrode 7 is arranged between a respective first fixed electrode 5a and a respective second fixed electrode 5b. The mobile mass 6 is suspended via springs 10 to anchorage elements 11.
The mobile mass 6 can move along an axis 13 that constitutes the preferential axis of detection of the linear accelerometer 1.
As shown in FIG. 3, the sensing element 2 can be schematically represented as a first capacitor C1 and a second capacitor C2 arranged in series, the capacitances of which are variable as a function of the distance between the mobile electrodes 7 and the fixed electrodes 5a, 5b, and hence as a function of the displacement of the rotor with respect to the stator. In particular, the first capacitor C1 is formed by the set of the first fixed electrodes 5a and by the set of the mobile electrodes 7, whilst the second capacitor C2 is formed by the set of the second fixed electrodes 5b and by the set of the mobile electrodes 7.
When the linear accelerometer 1 is subjected to an acceleration along the axis 13, the mobile mass 6 moves along said axis, and it consequently produces a capacitive unbalancing between the first capacitor C1 and the second capacitor C2. This capacitive unbalancing is detected by the conditioning electronics, which then supplies at an output the signal Vout.
In particular, the displacement of the mobile mass 6 arises even in the presence of a static acceleration (for example, the acceleration of gravity), generating a corresponding capacitive unbalancing, which is detected by the conditioning electronics. Accordingly, even in a rest condition, a non-zero acceleration is detected, having a value equal to that of the component of the acceleration of gravity along the axis 13.
Since in general it is desired to determine the displacements of the portable apparatus along three axes (corresponding to the three axes x, y and z of a set of Cartesian axes fixed with respect to the portable apparatus), the displacement detection device generally has three uniaxial linear accelerometers, each of which detects the component of the acceleration acting on the portable apparatus along one direction of detection. Alternatively, in an equivalent way, a single accelerometer provided with three axes of detection may be envisaged. In any case, three acceleration signals Ax, Ay and Az are generated, which represent the component of the acceleration along the x, y, and z axis, respectively.
The displacements of the portable apparatus are then determined by an appropriate processing circuit, which processes the acceleration signals Ax, Ay and Az, and in particular compares them with respective fixed acceleration thresholds. Exceeding of one of the acceleration thresholds indicates that the portable apparatus has undergone a displacement in the corresponding direction.
As previously described, due to the acceleration of gravity, the accelerometers have a non-zero output even in the absence of any acceleration imparted by the user on the portable apparatus. In particular, the acceleration signals Ax, Ay and Az due to the sole acceleration of gravity have different values according to the inclination of the portable apparatus, in so far as the components of the acceleration of gravity along the axes x, y and z, respectively, differ each time. Consequently, exceeding of the acceleration thresholds by the acceleration signals Ax, Ay and Az, in the presence of an acceleration imparted on the portable apparatus, depends upon the initial position (and in particular upon the orientation) of the portable apparatus.
This phenomenon entails disparity of operation according to the initial resting position of the portable apparatus, as determined by the user. Furthermore, this problem is aggravated by the fact that each user has a personal way of holding a portable apparatus, and the differences increase, for example, between right-handed users and left-handed users.
To solve the aforesaid problem, it has been proposed to set, in an initial step prior to displacement measurements, a reference position, corresponding to the resting position of the portable apparatus, and then to refer the detected displacements to said reference position, in a differential manner. A circuit diagram of a processing circuit implementing this solution is shown in FIG. 4.
In detail, the processing circuit, designated by 20, comprises three registers 14-16, three adders 17-19, and three threshold comparators 21-23. The registers 14-16 receive at input a respective acceleration signal Ax, Ay and Az from an accelerometer 25, and store an initial value thereof. The adders 17-19 each receive at input a respective acceleration signal Ax, Ay and Az and the output of a respective register 14-16. The three threshold comparators 21-23 receive at input the signal outputted by a respective adder 17-19, compare it with an acceleration threshold Ath, and supply at output a respective logic signal that is the result of the comparison. This logic signal represents detection of a significant acceleration along the corresponding axis. The acceleration threshold Ath is stored in a dedicated threshold register 25.
Following an appropriate external initialization command (designated by INIT in FIG. 4), the initial values of the acceleration signals Ax, Ay and Az in the resting position of the portable apparatus are stored in the registers 14, 15 and 16 (these initial values will be used as reference values). Then, the reference values stored in the respective register are subtracted from the acceleration signals Ax, Ay and Az each time detected by the accelerometer 25. In this way, the variation of the respective acceleration signal Ax, Ay and Az with respect to the resting position is provided at input to the threshold comparators 21, 22, 23, and hence the displacement detection does not (in theory) depend upon the initial position (i.e., upon the acceleration of gravity), but only upon the displacements imparted on the portable apparatus.
A drawback of the described processing circuit 20 is linked to the fact that it is necessary to automatically determine when new reference values are to be set in the registers 14, 15 and 16. Furthermore, once the reference position has been established, the user must maintain the portable apparatus in said position to ensure correct operation of the displacement detection device. In fact, the described circuit is not able to adapt automatically to the change of posture of the user, including for example the inevitable (in so far as involuntary) movements of the wrist that cause a variation in the portable apparatus orientation. Furthermore, in the worst case (i.e., when said involuntary movements are greater than the acceleration threshold), the involuntary movements could trigger an undesirable activation event. It is, consequently, evident that a solution of this type actually makes unnatural the use of functions activated by movement, which instead should simplify the man-machine interface.
Another proposed solution envisages setting the acceleration threshold each time the portable apparatus is displaced. This solution is, however, far from practicable, unless high computing power is available.